This invention generally relates to electrothermal guns and more particularly to a propulsion apparatus employing a chemical propellant augmented by transmission and deposition of electrical energy in the form of a plasma.
Various schemes have been investigated over the years to enhance the energy output of conventional guns and cannons. For example various liquid propellant and traveling charge concepts have been explored for the purpose of achieving higher performance levels. Various electrical gun schemes, electrothermal (ET) and electrothermal-chemical (ET-C) schemes, electromagnetic launchers and coil guns have also been proposed to attempt to improve the energy output of modern weapons.
Examples of these guns are disclosed in U.S. Pat. Nos. 4,907,487, 4,913,029, 4,715,261, 4,590,842, 4,711,154, 4,895,062, 4,555,972, and 4,640,180. Developments in electromagnetic launchers or rail guns are disclosed in U.S. Pat. Nos. 4,534,263, 4,840,106, 4,901,620, and 4,901,621.
The first two patents listed above, U.S. Pat. Nos. 4,907,487 and 4,913,029 both teach forming a low temperature plasma behind a projectile made in general of a high strength dielectric by vaporizing a fluidizable dielectric material such as polyethylene beads, water or lithium hydride and ohmically heating the plasma to accelerate the projectile through a gun tube lined also with a fluidizable low atomic weight material such as polyethylene. The polyethylene lining is simultaneously vaporized, augmenting the plasma pressure behind the projectile to achieve a muzzle velocity of on the order of 10 Km/second.
U.S. Pat. No. 4,715,261 similarly discloses a projectile accelerated by a plasma. However, in this case the plasma is supplied to the rear of the projectile through a capillary tube behind the projectile containing an ionizable dielectric. A discharge voltage is applied to the dielectric forming the plasma.
U.S. Pat. No. 4,590,842 discloses accelerating a projectile through a gun tube by sequentially injecting a pulsed high pressure plasma into the volume behind the projectile through spaced capillary injection tubes discharging into the gun tube at spaced intervals along the gun tube as it travels through the tube. The high pressure plasma is generated by vaporizing a dielectric lining in the capillary tubes.
U.S. Pat. No. 4,640,180 discloses augmenting a chemically propelled projectile by generating an electromagnetic field in the propellant gas during the combustion process. The propellant chamber is surrounded by a helical coil through which a high current is passed when chamber pressure rises to a preset value to produce eddy currents in the plasma, further heating the plasma creating a boost in chamber pressure to further accelerate the projectile. Alternatively, an electrical arc is generated in the propellant chamber during the chemical propellant combustion which ohmically further heats the propellant gases to achieve the same result.
U.S. Pat. No. 4,711,154 discloses a plasma injector for injecting a combustible fuel into the chamber behind a projectile which contains an oxidizer. The fuel injection rate into the oxidizing chamber is controlled by the electrical current pulse applied to the plasma injector. The plasma injector is a capillary tube which contains dielectric beads. An electrical pulse creates an arc through the beads (polyethylene), vaporizing them and producing a high pressure plasma of partially ionized ethylene which is injected into the oxidizer in the propellant chamber. The oxidizer, preferably hydrogen peroxide, then combusts the ethylene fuel to produce the propellant gas to accelerate the projectile.
U.S. Pat. No. 4,895,062 discloses an improved augmented plasma gun similar to that described in U.S. Pat. No. 4,711,154 but having an additional second fuel adjacent the oxidizer and in front of the capillary tube containing the dielectric beads which form the initial plasma upon imposition of a current pulse through the capillary tube. A fuse wire longitudinally disposed in the capillary chamber is vaporized by the current Pulse passing through it creating the heated plasma of ionizing gas, maintaining an electrical current path through the fuel in the capillary tube. This produces a jet of ionized gas injected into the fuel and the oxidizer chambers to begin the combustion process.
U.S. Pat. No. 4,555,972 discloses an electromagnetic launcher in tandem with a chemical projectile accelerator. The projectile accelerator is a conventional rifled barrel and cartridge chamber arrangement. The projectile is a dielectric saboted projectile preferably having a metallic armature on its rear end. Alternatively, a plasma armature is formed just behind the projectile. The rails of the electromagnetic launcher portion are pulsed with a high current source as the projectile enters one end of the launcher. The current pulse creates an arc behind the projectile passing through the armature, further accelerating the projectile through the bore. The armature in the rear of the projectile and the rails constitute essentially a one turn linear motor.
U.S. Pat. No. 4,930,394 teaches a hybrid electrothermal chemical propulsion system for a projectile wherein fluid propellant components are reacted and the gases produced thereby are forcibly conducted past a plasma arc electrode arrangement which couples the thermal energy of the arc discharge into the gases streaming by.
One of the major problems encountered with the electrothermal gun concepts is that the generation, storage, and transfer of the electrical energy necessary to achieve the desired performance levels requires heavy and bulky components which are impractical. The electrothermal-chemical (ET-C) propulsion concepts are particularly attractive because of reduced bulk and the total energy release may be controlled or modulated by a metering or other control of the electrical energy input a described in the patents discussed above.
However, the practitioners of the art have to date relied on the creation of plasmas, arcs, and other electrical discharges for the purpose of exciting, ejecting, modulating and combusting various chemical mixtures, compounds and propellant formulations, usually as liquids, slurries, etc .
Army ET-C weapon tests and liquid propellant gun tests have so far both exhibited excessive pressure variations and excursions or spikes during high energy firings. These excursions or spikes have led to permanent deformation of the gun tubes and uncontrollable impulses delivered to the projectiles resulting in their structural failure and unacceptable muzzle velocity variations, These test results are but symptoms of the actual lack of combustion control exhibited by the various ET-C schemes used to date when coupled with the various working fluids used. It is believed that certain schemes such as that disclosed in U.S. Pat. Nos. 4,722,154 and 4,895,062 discussed above, will not scale in a straight forward nor practical fashion when applied to much larger geometries such as high performance tank cannons and large artillery systems. The reasons for this are not entirely clear at this time.
One reason identified is the requirement for a multiplicity of the contained high pressure plasma generation devices in such systems. These devices must structurally be able to withstand very high internal pressures on the order of 50,000 to 100,000 psi, necessitating heavily constructed and bulky structures. Such structures must also exhibit virtually identical electrical load characteristics and therefore energy coupling characteristics both during the single ballistic cycle and as well as shot to shot in order to uniformly and repeatably ignite and modulate the combustion of an inherently unstable and chaotic combustion process such as is presented by liquid propellants and working fluids.
This is especially true for the bipropellant systems where the mixing process is also initiated and maintained or modulated by the high energy plasma and the excited fuel and oxidizer species carried along and entrained in the discharge. Accordingly, there is a need for an improved electrothermal chemical scheme to achieve improved performance in large scale applications such as tank cannons and artillery.
The present invention draws from the combined technologies of solid propellant propulsion and electrothermal propulsion, neither of which teach such a hybrid combination as will be shortly described.
In solid propellant propulsion devices one or more energetic and nonenergetic materials are combined in such a fashion as to produce a solid grain of propellant with specific structural and chemical energy characteristics as well as contained chemical energy release characteristics. These characteristics can be controlled by the geometry, chemistry, and environment in which the grain is used. The environment includes initial propellant temperature, igniter system output energy and duration, as well as resultant initial pressure, chamber volume, bore diameter, projectile mass, and the travel distance of the projectile within the gun bore. This technology represents a very mature technology base. It is in fact the baseline to which all new propulsion concepts are measured.
The Electrothermal (ET) and electrothermal chemical (ET-C) systems as applied to large guns are both new technologies. These devices utilize the electrical output of rotating machinery or some other prime electrical source through a pulse forming network consisting of capacitors and inductors energizing the cathode of the system. A dielectric breakdown plasma is directed to a chamber containing an inert working fluid (as in the case of pure ET) which is vaporized to provide gas pressure to eject or propel a projectile. Also in the case of pure ET the resulting device has the serious drawback at this time of being very bulky due to the excessive size of the required electrical power supply. In the case of the ET-C systems, the electrical power supply promises to be much smaller since not all of the energy required to propel the projectile need come from the electrical energy source.